THIS invention relates to a method and apparatus for the identification of objects, and in particular for the identification of a plurality of remote electronic identification tags by a reader using electromagnetic communication means.
Numerous different types of electronic tags, which are typically in the form of transponders, are attached to physical objects such as goods, equipment, people, animals and the like. These tags are programmed to contain identity data, which is used to electronically identify the tagged objects via interrogation by a reader. The read data is typically arranged to be transferred to a computer system for tallying and reading the identity of those objects which are in the interrogation area of the reader.
A large number of patents already cover the situation of a reader and a plurality of transponders with the communication between the reader and the transponders being based on acoustic or electromagnetic radiation principles. In most cases, the prior art protocol requires the transponders to have a unique identification number and the ability to receive and decode a transmission containing a data stream from the reader and match it to all or components of its unique identification number.
Another class of prior art is an identification system that needs neither a unique identity nor a multi-bit receiver and decoder for the transponder, but which relies on the reader communicating with the transponder at the correct time after it has finished its transmission, if the transponder was successfully identified. The communication can be single bit in format, and the message is communicated by the timing of the communication after the completion of transmission, the communication being as simple as disturbing the energising field from the reader at the correct instant. Such a system is described in South African patent 92/0039, which requires the reader and the transponder whose identity is being determined to be synchronised and to remain synchronised after communication has been completed so that the reader may respond at the correct time after the transmission has been completed.
A further category of prior art covers an identification system that uses a selection process to isolate a single transponder so that the transponder""s data can be received without being corrupted by transmissions from other transponders. U.S. Pat. No. 5,751,570 describes such a system. A collision signal is initially sent by the reader to place all transponders in to a dead state. Each transponder calculates a random dead state after receiving the collision signal, during which they do not transmit their data. At the end of the dead state each transponder will transmit its data again. If the reader receives data from only one transponder it sends an occupied or busy signal which place, all the transponders except the one busy transmitting into an idle state. The transponders in the idle state no longer transmit data. After the reader has received the complete data from the individual transponders the reader transmits an acknowledge signal which places the identified transponder into a passive or idle state in which it ceases transmission of its data. The rest of the transponders in idle state are reactivated and they recalculate a random dead time. The steps above are repeated until all the transponders have been identified. The drawback of this system is that if the electromagnetic field is changed to send the busy signal, the changes in the electromagnetic field may cause the receiver to misread the data from the transponder because the transponder signal is very much smaller than the changes in the electromagnetic field
A similar system is described in U.S. Pat. No. 5,124,699, where changes in frequency are used to send signals to the transponders, Initially a frequency shift is sent to the transponders to start a selection procedure that will isolate an individual transponder. The transponders calculate a random delay after which they transmit a starting block. Due to the random delay, only a few transponders might transmit a starting block at the same time. When the reader receives a starting block it changes the transmitter frequency which causes the transponders not transmitting a starting block to go into a passive state. The remaining transponders transmit their unique code. If the reader detects that more than one transponder is transmitting its code then the reader signals an error by changing the transmitter frequency which causes these transponders to recalculate a random delay again and to follow the steps above. The transponders in the passive state remain so. This selection process will eventually yield an individual transponder. Once the reader has identified the individual transponder it is placed into a passive state by another frequency change which also causes the remaining unidentified transponders to restart the selection process again. This system requires complex tuned circuitry in the transponder so that the different frequencies can be detected
According to a first aspect of the invention there is provided a method of identifying objects by an interrogator, comprising the steps of:
transmitting an interrogation signal from the interrogator to the objects;
transmitting from each object to the interrogator an identification signal having predetermined indicator characteristics in response to the interrogation signal;
receiving the identification signals from the objects at the interrogator and substantially contemporaneously determining at the reader if any identification signal has been individually and correctly received on the basis of the indicator characteristics;
substantially contemporaneously transmitting from the interrogator a common re-transmit or interrupt signal in the event of any identification signal not being individually and correctly received; and
ceasing signal transmission from an object if that object completes its signal transmission without receiving an interrupt signal front the interrogator during such transmission.
Preferably, the method includes the step of temporarily suspending signal transmission from an object if that object is transmitting its identification signal at the time it receives the interrupt signal from the interrogator.
In a preferred form of the invention, the method comprises the further steps of continuing so to receive all identification signals at the interrogator, and to transmit the interrupt signals, until no further identification signal is individually and correctly received for a time period sufficient to ensure that all identification signals from objects have been individually and correctly received by the interrogator.
In one form of the invention, the method may comprise the steps of continuously transmitting the interrogation signal and transmitting the interrupt signals at a time interval which defaults to less than the average transmission time of a valid identification signal in the absence of the receipt of an identification signal at the interrogator.
Conveniently, the method includes the step of transmitting at random time intervals an identification signal from each object which has not ceased its signal transmission and allowing each object which has ceased its transmission to recommence transmission in response to a reset event.
The reset event may include the absence of or variation in the interrogation signal for a predetermined minimum time period.
Advantageously, the method further includes the steps of transmitting a disable signal from the interrogator, receiving the disable signal at at least one of the objects, and setting a memory element in the object preventing it, only after the object has ceased signal transmission, from responding to any subsequent interrogation signal for a minimum predetermined stand-off time period.
The method may include the still further steps of transmitting an enable signal from the interrogator, receiving the enable signal at at least one of the objects, and resetting the memory element in the object to enable the object and allow it to respond to the same or subsequent interrogation signal in the manner described above.
Advantageously, the enable and disable signals are at least initially transmitted prior to the possible transmission from any object of an identification signal, and the enable and disable signals also serve as interrupt signals
Typically, the predetermined indicator characteristics of the identification signals are identical in form and have a predetermined duty cycle, and include a fixed length data stream preceded by an initial header and including a data component and a checksum component, with the transmission of a signal from an object commencing with the same header, and the interrogator being arranged to accept the start of an identification signal only if there has been no received transmission immediately prior to the receipt of such header.
The indicator characteristics may further include the transmission of the identification signal in a Manchester form modified to combine transmission clock rate and the data stream to produce a 50% duty cycle at a minimum operational bandwidth.
The invention extends to an identification system comprising an interrogator and a plurality of object-based responders, the interrogator including transmitter means for transmitting an interrogation signal to the responders, receiver means for receiving identification signals from the responders, and processor means for determining the individual and correct receipt of an identification signal, each responder comprising a receiver for receiving the interrogation signal, generator means for generating the identification signal, a transmitter for transmitting the identification signal back to the interrogator, a detector for detecting the presence of an interrupt signal from the interrogator, and control means responsive to the detector and being arranged to cease signal transmission from the responder if the responder completes transmission of the identification signal without receiving an interrupt signal during such. transmission.
Preferably, the control means is arranged temporarily to suspend transmission of an identification signal from the responder if the responder is transmitting its identification signal at the time it receives the interrupt signal.
Advantageously, the generator means comprises a first memory means for storing identification data, an oscillator, a Manchester encoder for deriving encoded identification data from the identification data and the oscillator, and a modulator driven by the Manchester encoder to derive an identification signal, and the transmitter and receiver comprise an antenna coupled to an RF module for performing backscatter modulation of the identification signal.
Typically, the interrogator is arranged to continue receiving identification signals, and transmitting the interrupt signal, until no further identification signal is individually and correctly received for a time period sufficient to ensure that all identification signals have individually and correctly been received by the interrogator.
The identification system may include processor means for generating the interrupt signals and for generating enable and disable signals for respectively enabling and disabling the responders, and each of the responders include second memory means arranged to be set by a disable signal for preventing the responder from responding to any subsequent interrogation signal from the interrogator for a minimum predetermined time period, only after it has ceased signal transmission, and to be reset by all enable signal for allowing the responder to respond immediately to an interrogation signal.
According to a further aspect of the invention there is provided a responder for an identification system of the type comprising an interrogator and a plurality of responders, each responder comprising a receiver for receiving an interrogation signal, first memory means for storing identification data, an oscillator a modulator for deriving a modulated identification signal from the identification data and the oscillator, and a transmitter for transmitting the identification signal back to the interrogator, the responder further comprising a detector for detecting the presence of an interrupt signal from the interrogator, and signal ceasing means arranged to cease signal transmission from the responder if the responder completes transmission of the identification signal without receiving an interrupt signal from the interrogator during such transmission.
The responder advantageously includes signal suspending means responsive to the detector and being arranged temporarily to suspend transmission of an identification signal from the responder if the responder is transmitting its identification signal at the time it receives the interrupt signal.
Typically, the signal suspending means and the signal ceasing means are incorporated in control logic circuitry, the responder further including a random timer connected to the control logic circuitry for enabling identification signals to be repeatedly re-transmitted at varying random time intervals from the responder until such time as it has completed the transmission of an identification signal without being interrupted by an interrupt signal.
Conveniently, the responder includes second memory means arranged to be set by a disable signal for preventing the responder, only after the responder has ceased signal transmission, from responding to any subsequent interrogation signal for a minimum predetermined time period.
The second memory means may be arranged to be reset by an enable signal for allowing the responder to respond to a subsequent interrogation signal after the responder has ceased signal transmission.
Typically, the second memory means comprises a register responsive to disable signals and a memory module responsive both to the register and to the signal ceasing means for allowing the responder to complete the transmission of an identification signal prior to being disabled.
Advantageously, the responder further includes a Manchester encoder for deriving encoded identification data from the identification data and the oscillator, for receipt at the modulator, and the transmitter and receiver comprise an antenna coupled to an RF module for performing backscatter modulation in response to an interrogation signal, the Manchester encoder being responsive to an output enable signal signifying completion of the transmission of the identification signal.
According to a still further aspect of the invention there is provided an interrogator for an identification system of the type comprising an interrogator and a plurality of responders, the interrogator including transmitter means for transmitting an interrogation signal to the responders, receiver means for receiving identification signals from the responders in response to the interrogation signal, first processor means for substantially contemporaneously determining the individual and correct receipt of an identification signal, signal generator means responsive to the processor means for generating substantially contemporaneously an interrupt signal in the event of any identification signal not being individually and correctly received from any one or more objects, and for causing the interrupt signal to be transmitted sufficiently quickly to suspend signal transmission from any such object whilst it was not individually and correctly transmitting its identification signal.
Preferably, the first processor means is arranged to continue receiving and processing all identification signals, and the signal generator means is arranged to continue generating interrupt signals, until no further identification signal is individually and correctly received, for a time period sufficient to ensure that all identification signals from objects have been individually and correctly received.
Typically, the transmitter is arranged to transmit the interrogation signal continuously, and the signal generator means is arranged to transmit the interrupt signal at a time period which defaults to less than the average receipt time of a valid identification signal in the absence of the receipt of an identification signal at the interrogator.
Conveniently, the signal generator means is arranged to generate enable and disable signals for transmission via the transmitter, the disable signal being arranged to set memory elements in the objects preventing them, only once they have ceased signal transmission, from responding to any subsequent interrogation signal for a minimum predetermined stand-off time period, and the enable signal being arranged to reset the memory elements to allow the objects to respond to a subsequent interrogation signal.
The receiver may include a receiving antenna and a quadrature receiver, and the first processor means may include a signal processor for processing baseband components of the identification signal derived from the quadrature receiver into a reconstructed Manchester data signal, and an error checking microprocessor for decoding and error checking the Manchester signal at least on the basis of duty cycle, clock rate, data stream length and checksum calculations.